Precise non-contact measurement of the deformation or displacement of objects is critical when active or passive compensation for the deformation or displacement is required. For example, large parabolic dishes used for communication, radar or telescopes are susceptible to many natural influences which distort the shape of the dishes. Some of these natural influences include wind, gravitational forces which can cause the dish to sag depending on the orientation of the dish, and temperature variations in the dish which can distort the shape of the dish, etc. To compensate for these effects, several solutions may be employed. One such solution is to use mechanical actuators on a segmented dish with an array of detectors mounted in different locations on the dish. As the detectors sense a change in the shape of the dish, the actuators respond by moving segments of the dish to correct the shape of the dish. A large disadvantage of this technique is the amount of cables and other electrical components which need to be mounted to the dish to perform the compensation. Another solution involves using mathematical techniques to correct the received signal as reflected by the deformed surface. This technique is of limited usefulness since the distortions in the dish are typically not uniform and cannot always be accurately modeled. These measurements can also be applied to other structures where deformations are studied, such as building surfaces, airplane surfaces, space shuttle surfaces, and surfaces of automobiles. In each case, the amount of surface deformation must be accurately measured so that the correct amount of compensation can be applied. These measurements can effect the design of wings on an airplane or the rear spoiler on a racing car, for example.
Other techniques for measuring deformations in the surface of objects include using laser range finders which provide precise measurement information. Typically it is cost prohibitive to use many of these laser range finders to simultaneously measure many points on the surface. More practically these range finders are used for individual sequential measurements at different points on the surface of the object. Hence, the measurements are not made simultaneously. Also, these laser range finders are sensitive to temperature changes in the atmosphere along the z-axis in FIG. 1. Temperature variations between the left side of FIG. 1 and the right side of FIG. 1, or bulk temperature changes along the entire path L can severely compromise the measurement accuracy of these laser trackers.
Another measurement technique projects a fringe pattern on a detector which is mounted to the surface to be measured. As the surface deforms, the detector moves and sweeps across the fringe pattern. By detecting the changes in the light intensity, the deformation of the surface can be determined. Although this technique provides a relatively inexpensive way to simultaneously detect relative displacement of a surface, it is sensitive to temperature variations in the atmosphere as well, but in the direction of the x-axis in FIG. 1. In other words, the temperature sensitivity of this technique is to temperature variations in the direction displacements are being measured. Therefore, this measurement technique is sensitive to temperature gradients. Those gradients result in shifts in the fringe pattern at the detector independent of the relative motion of the detector and the fringe pattern. Note that this technique is not sensitive to temperature variations along the z-axis as shown in FIG. 1.
These optical techniques are susceptible to temperature variations in the atmosphere because those temperature changes cause index of refraction variations. These index of refraction variations cause light traveling through the atmosphere to bend. The amount of this bending depends on the severity of the refractive index variations. The measurement techniques discussed above do not compensate for these refractive index variations. Thus, the measurement result is not as precise as it would be without refractive index variations in the atmosphere. Therefore, non-contact techniques for measuring surface deformation or distortion cannot identify whether the distortion is due to the wind, gravity, or atmospheric effects affecting the measurement equipment.
The present invention provides a method and apparatus for compensating for atmospheric effects that typically plague measurement equipment. The technique is useful in precise non-contact measurement of surface distortion without adding the uncertainty of refractive index changes in the atmosphere. The technique could also be used in precision land surveying, to aid in the building of a linear accelerator, or any situation where precise straightness measurements are required. The technique may be used to compensate for measurement uncertainty due to atmospheric refractive index effects.